We say that it is 12 noon, or 12 o’clock midday, when the Sun is at its highest in the sky. In this diagram the Sun will appear at its highest from point (C) on the Earth and so it is 12 o’clock midday at point (C)
At the same instant, point (D), which is further west on the Earth, still sees the Sun climbing higher in the sky. Viewed from point (E) there are still a few hours to go before midday. It is mid-morning at point (D).
At point (B), a more easterly point than (C), the Sun has already passed its highest position in the sky. It is the middle of the afternoon at point (B).